There have been proposed an image forming apparatus of an electrophotographic type which has a plurality of photosensitive drums arranged in parallel, chargers charging the photosensitive drums, developing rollers supplying toners to the photosensitive drums, a conveyance belt conveying a recording sheet between the belt and the photosensitive drums, and the like. The related-art image forming apparatus is configured to switch a color mode, in which a color image is formed using all the photosensitive drums, and a monochrome mode, in which the developing rollers except for the developing roller for black are separated from the corresponding photosensitive drums and a monochrome image is formed using only the photosensitive drum for black provided at the most downstream side in a conveyance direction of the recording sheet.
Further, there have also been proposed another image forming apparatus of an electrophotographic type which has cleaning rollers for collecting toners attached (remaining) on the photosensitive drums and one constant voltage source for applying a bias voltage to the cleaning rollers.